The invention involves a compact, rugged coating monitor. Corrosion is a wide-spread problem that affects nearly all industry and government sectors. A recent report determined that the direct cost of corrosion in the United States to be 3.1% of the Gross Domestic product (GDP) [Gerhardus H. Koch, Michiel P. H. Brongers, Neil G. Thompson. Y. Paul Virmani, Joe H. Payer, “Corrosion Costs and Preventive Strategies in the United States,” Report by CC Technologies Laboratories, Inc. to Federal Highway Administration (FHWA), Office of Infrastructure Research and Development, Report FHWA-RD-01-156, September 2001]. This corresponds to $300B annually or $1000 per person. This figure includes only the direct costs (e.g., corrosion prevention, corrosion inspection, and replacement or refurbishment of corroded structures). The indirect costs (e.g., lost productivity, taxes, and overhead) were conservatively estimated to be equal to the direct costs.
Paint coatings are the primary means of corrosion protection for most structures and they can be very effective. However, paint coatings are only temporary; they weather, absorb moisture, blister, become scratched or undergo other mechanical damage. Even fresh paint coatings can exhibit pinholes, holidays, or other coating defects that can adversely affect corrosion protection. Thus, there is a need for corrosion sensors to assess the health and effectiveness of paint coatings, especially on critical structures and equipment.
Although many corrosion sensors have been proposed and developed [G. D. Davis, C. M. Dacres, and L. A. Krebs, “In-Situ Corrosion Sensor for Coating Testing and Screening,” Materials Performance 39(2), 46 (2000); G. D. Davis, C. M. Dacres, and L. A. Krebs, “EIS-Based In-Situ Sensor for the Early Detection of Coating Degradation and Substrate Corrosion,” Corrosion2000, Paper 275 (NACE, Houston, Tex., 2000); G. D. Davis, C. M. Dacres, and L. A. Krebs, “EIS-Based In-Situ Sensor for the Early Detection of Coating Degradation and Substrate Corrosion,” Corrosion2000, Paper 275 (NACE, Houston, Tex., 2000); J. Green, M. Jones, T. Bailey, and I. Perez, Process Control and Sensors for Manufacturing, R. H. Bossi and D. M. Pepper, ed., (SPIE—The International Society for Optical Engineering, Bellingham, Wash., 1998), p. 28; V. S. Agarwala, Corrosion96, Paper 632, NACE, Houston, Tex., 1996; R. G. Kelly, J. Yuan, S. H. Jones, W. Blanke, J. H. Alor, W. Wang, A. P. Batson, A. Wintenberg, and G. G. Clemena, Corrosion97, Paper 294, NACE, Houston, Tex. 1996; J. Zhang and G. S. Frankel, in Nondestructive Characterization of Materials in Aging Systems, MRS Symp. Series, Vol. 503, R. Crane, J. Achenbach, S. Shah, T. Matikas, P. Khuri, and L. Yakub, eds., (Materials Research Society, Warrendale, Pa., 1998), p. 15; R. E. Johnson and V. S. Agarwala, Corrosion97, Paper 304, NACE, Houston, Tex., 1997; L. D. Stephenson, A. Kumar, J. Hale, and J. N. Murray, “Sensor System for Measurement of Corrosion Under Coatings,” Mater. Perf. 48(5) 36 (May 2009)], many are not suitable for monitoring coating health. A major disadvantage of many of these, such as galvanic couple sensors (U.S. Pat. No. 5,306,414, U.S. Pat. No. 5,437,773, U.S. Pat. No. 5,243,298, U.S. Pat. No. 6,809,506, U.S. Pat. No. 4,380,763, U.S. Pat. No. 6,683,463, U.S. Pat. No. 7,313,947, U.S. Pat. No. 5,338,432, and U.S. Pat. No. 5,310,470) and many fiber optic corrosion sensors (U.S. Pat. No. 5,299,271, U.S. Pat. No. 7,228,017), is that they are more properly considered corrosivity sensors; that is, they detect degradation of a sensor element and not degradation of the structure of interest. As such, they measure only how corrosive the environment is and provide no direct information on the condition of the coating or the structure. Furthermore, they are consumed and have a limited lifetime and can provide no information concerning any environmental degradation prior to installation. A second disadvantage of many sensors is that they need to be embedded into the structure. This limits them to new construction and poses important issues on the effect of the sensor on structure properties and data acquisition/transfer. These sensors cannot inspect existing structures and cannot be replaced if damaged or past their useful lifetime. The electrochemical impedance sensor approach developed here has neither of these critical disadvantages. The technology is suitable for determining coating health and detection of damage under paint coatings.
The coating monitor of the present invention is a compact and rugged integrated detection and reporting system that uses electrochemical impedance spectroscopy (EIS)-based corrosion sensors and mini-potentiostat elements. Corrosion sensors for military and civilian applications are most useful when the devices do not require invasion into the coating being monitored for protection or embedding within the structure. The coating monitor of the present invention uses conductive tape sensors to allow EIS measurements to be taken without remote electrodes. Using that engineering approach allows the coating monitor to be applied at times and in locations needed with the flexibility to remove it without decommissioning the structure or vehicle to which it is attached, or removing part of the monitored structure. Its small size and ruggedness are a huge commercial advantage. A patent search found a number of patent documents that approached some aspect of coating monitoring from an EIS sensor perspective. Documents of particular note include U.S. Pat. No. 7,477,060 ('060 Yu, S. Y. et al) and published application US20080150555 ('555 Wang, D. et al), which has its sensors on the surface or embedded in a flexible substrate integrated into the monitored structure, and U.S. Pat. No. 6,911,828 ('828 Brossia, C. S. et al), which has arrays of sensor pins in contact with the structure's coating. Also of interest is U.S. Pat. No. 7,088,115 ('115 Glenn, D. F. et al), for an EIS system designed specifically for uncured concrete.
With respect to the '060 patent, the specification discusses the feasibility of attaching the sensors with adhesives but this feature is not included in the claims of the patent. In the published application, '555, an optoelectronic backbone communicates data from the sensors to a control device.
Similarly, the '828 patent describes a sensor array in the form of sensor pins in contact with the coating being monitored and a data interrogation device positioned in proximity to the sensor array.
The '115 patent is directed specifically at detecting defects in uncured concrete before the curing process. It is much more limited in application than the '060 or '828 systems.
The DACCO SCI, INC., portfolio discloses permanent or handheld sensors that use EIS to detect moisture and other changes in coatings U.S. Pat. No. 5,859,537 ('537 Davis, G. D. et al), U.S. Pat. No. 6,054,038 (038 Davis, G. D. et al), U.S. Pat. No. 6,313,646 ('646 Davis, G. D. et al) and U.S. Pat. No. 6,328,878 ('878 Davis, G. D. et al). The '537 patent is directed to an in situ sensor suitable for coated metal structures. The sensor comprises conductive ink and is permanently applied to the topcoat being monitored. The '038 patent teaches the corrosion sensor as a handheld device comprising a metal. The '646 patent teaches the use of two hand-held electrodes to detect moisture absorption, corrosion, and adhesive bond degradation. The '878 patent teaches the use of a pair of conductive foil adhesive tapes, one with a conductive adhesive and one with a nonconductive adhesive, to determine coating or substrate degradation. All of these inventions use a separate bench-top or similar-sized potentiostat to be connected and to acquire the EIS measurements; thus they are not suitable for remote or unattended operation.
An example of a prior art potentiostat would be the Gamry Reference 3000 potentiostat that is 20-cm×23-cm×30-cm and weighs approximately 6 kg. FIG. 1 shows a block diagram of a generic potentiostat 10 comprised of an ac voltage generator 12; a galvanometer 14 to measure the current (magnitude and phase) induced by the said ac voltage; a means 16 to make electrical connection to the specimen being measured; a means 18 to make electrical connection to reference and counter electrodes immersed into an electrolyte along with the specimen; a means 20 to convert the current measurement into an electrochemical impedance measurement (magnitude and phase); and a means for input/output 22.